The present invention is directed to an accelerometer, especially one capable of measurement of very high acceleration and deceleration rates. The accelerometer is particularly useful for measurement of extreme forces such as those caused by impacts of moving bodies contacting another moving or a stationary body.
Accelerometers are devices widely used for applications as diverse as vibration monitoring, appliance control, joysticks, industrial process control, space launches, satellite control, and many others. A use suggested by one manufacturer, without further elaboration, is for sports diagnostic devices and systems. Many different types are available but all depend on measuring the inertial lag of some element during a positive or negative velocity change in a moving article. In one common type the element subject to inertial displacement may act as one plate of a capacitor or may be a moving plate between two fixed plates of a capacitor. The amount of inertial displacement of the sensitive element is extremely minute but this can be accurately measured by state-of-the-art circuitry and calibrated to indicate gravitational force measurements. Different types are available to measure from relatively low to relatively high g forces. Often these devices will be in the form of very small integrated circuits. These are available from a number of manufacturers. Without intending to endorse any specific product or supplier, exemplary accelerometers might be Types ADXL 150/ADXL 250 available from Analog Devices, Norwood, Mass. or Types MMA1201P or MMA 2200W available from Motorola, Inc., Denver Colo. These are capacitor types that will measure forces up to about 50 g maximum. One problem has been the lack of availability of accelerometers to measure very high forces; e.g., in the range of 100 g and above. The present invention ably serves that need.
The present invention is directed to an accelerometer and to some specific applications of the accelerometer. The accelerometer is suitable for general use but is particularly well adapted for measurement of extreme acceleration and deceleration rates that may be in excess of 100 times normal gravitational force. While there is no limit implied in the possible applications of the instrument, specific examples will be given later in which the accelerometer is used in combination with a baseball bat to assist coaching of a hitter.
The accelerometer of the invention is based on the principle of inertial lag of a cantilevered optical fiber or fibers when the article to which the device is coupled is set into motion. This inertial lag, and the corresponding direction of motion of the article, are sensed by an array of photosensors receiving light transmitted from the fiber or fibers.
For purposes of ease of description it will be assumed that a single optical fiber is used. However, this should not be regarded as a limitation. A specific example will be given later showing how multiple fibers might be employed.
The invention requires a constant output light source to the optical fiber. This source may conveniently be a light emitting diode close coupled to the fiber. Appropriate circuitry well known to those skilled in the art assures constant current flow to and light output from the diode. Other light sources giving a constant output can be equally suitable. The optical fiber may be of any material commonly used for this purpose. While a clad plastic fiber is preferred, this is not essential. Glass fibers, clad or unclad, are also suitable. One end of the optical fiber is adjacent the light source and generally fixed in position. A fixed anchor point holds the fiber near the opposite end. However, a short cantilevered and unsupported portion of the fiber extends beyond the anchor point. It is this portion that is sensitive to inertial lag during movement. The cantilevered portion is preferably weighted to increase the mass subject to the inertial force.
Light emitted by the optical fiber (or transmitter) is detected by a photoreceptor array. This will have a plurality of photosensors that will output information both as to magnitude and direction of the transmitter deflection. The individual photosensors are preferably masked in a novel manner to effect a desired current output vs deflection relationship. The masking can be readily adapted to product a linear output curve or a curve of any other advantageous type; e.g., logarithmic.
The individual photosensors of the photoreceptor array are arranged about a central or neutral point. The relationship of the cantilevered section of the optical fiber transmitter and photoreceptor array may be adjusted so that the fiber is aimed directly at the neutral point; e.g., when the accelerometer is suspended with the transmitter oriented downward.
The individual photosensors are preferably coupled to a multiplexing circuit that will sequentially and repeatedly sample the output of the photoreceptor array. The sequencing is controlled by a precision timer. An output circuit conditions the signal from the multiplexing circuit. Here the multiplexer output is preferably changed from an analog to a digital signal. This conversion can result in a significantly improved signal to noise ratio as well as providing a direct computer input.
A receiver then processes the signal from the output circuitry. This contains circuitry and software to record or display acceleration, force, and position data sensed by the accelerometer. A timing circuit in the receiver is controlled by a phase lock loop to be in synchronization with the timing circuit of the accelerometer. The receiver and output circuitry may be hard wired to each other. Alternatively, the data from the output circuitry may be coupled to a transmitter that sends it to a remote receiver.
It is thus an object of the invention to provide an accelerometer that is simple in construction and well adapted to measure an extreme range of acceleration rates.
It is also an object to provide an accelerometer based on inertial deflection of a cantilevered optical fiber.
It is a further object to provide a masked photoreceptor array to determine magnitude and direction of deflection of the cantilevered portion of the optical fiber during movement of an object to which the accelerometer is associated and thus provide acceleration and direction of movement data.
It is yet an object to instrument an athletic implement with the accelerometer to provide real time data to assist in coaching an athlete.
These and many other objects will become readily apparent to those skilled in the art upon reading the following detailed description taken in conjunction with the drawings.